1. Field of the Invention
The present invention concerns fiber optic gyroscopes, and a method of operating a fiber optic gyroscope.
2. Discussion of the Known Art
Gyroscopes are used in inertial navigation systems as a means for measuring a rate of rotation of an object on which the gyroscope is mounted, with respect to a known axis of the gyroscope. Navigation systems installed on boats and on aircraft use gyroscopes to detect instantaneous changes in the orientation of the vehicle. The systems also typically include accelerometers for detecting changes of the vehicle's speed, and information provided by the gyroscopes and the accelerometers enables the system continuously to determine a present position of the vehicle with respect to a known starting location.
So-called fiber optic gyroscopes or “FOGs” have replaced many of the prior mechanical gyroscope configurations and provide greater accuracy and reliability in high performance commercial and military applications. Operation of a fiber optic gyroscope relies on the so-called Sagnac effect. Basically, light from a common source is divided into separate light beams that are directed to propagate in opposite directions through an optical fiber coil. After exiting from opposite ends of the coil, a relative phase difference between the beams is detected, and the difference is used to determine a rate at which the coil is rotating about its axis. See, H. Lefevre, The Fiber-Optic Gyroscope, Artech House (1993) at pages 5–25, and incorporated by reference.
FOG systems may be classified in either one of two general categories, viz., closed loop or open loop. In a closed loop system, a feedback path is defined so as to maintain the phase difference between the light beams constant after the beams exit the ends of the fiber coil. The amount of feedback needed to maintain the fixed phase relation is therefore indicative of the rate of rotation of the coil about its axis. Closed loop FOG systems have a disadvantage in that the range over which accurate rotation rate measurements can be obtained is limited by the ability to produce sufficient feedback to maintain the constant phase difference as the coil's rotation rate approaches a certain threshold. See, A. Tebo, High-Performance Fiber Optic Gyros and Their Future, Part 2, SPIE—The International Society for Optical Engineering (2000), at pages 2–3.
By contrast, open loop FOG systems calculate the rotation rate by way of amplitude measurements taken along an interference curve which results when the two exiting light beams are recombined. Conventional open loop FOGs have a limited phase measurement range which is bounded by {−90 degrees≦R≦+90 degrees} on the interference pattern, wherein R is the Sagnac-induced phase shift. As the rotation rate increases and the Sagnac shift approaches the mentioned limits, a determination of the rate becomes much less certain and ambiguities arise. This drawback limits the range over which rate measurements can be obtained reliably with the conventional open loop FOGs. In addition, the FOGs cannot distinguish between a detected rotation rate near zero, and a detected rate that happens to induce a Sagnac phase shift which is more than 360 degrees from zero when the system is initially turned on.
U.S. Pat. No. 5,052,808 (Oct. 1, 1991) discloses a rotation sensing interferometer of the closed loop type. Measurements are taken at points near +90 degrees, −90 degrees, +270 degrees and −270 degrees on an interference curve. Rotation-induced phase shifts then serve to produce feedback for keeping the measurement points at the +90, −90, +270 and −270 degree phases.
U.S. Pat. No. 6,256,101 (Jul. 3, 2001) relates to an open loop FOG for measuring high rates of rotation. The patented gyroscope has a drive circuit operative to adjust the phases of two light beams exiting a fiber optic coil once a measured rotation rate of the gyroscope exceeds a certain threshold.